1. Field of the Invention
The present invention relates to a cathode ray tube device and, more particularly, to a cathode ray tube device in which alternating electric field radiated from the deflection yoke is diminished.
2. Description of the Related Art
Referring to FIG. 4, a known CRT device has a funnel portion 1 which includes a neck portion 1a, a cone portion 1b and a funnel main body portion 1c which has a high voltage anode button 1d and is frit sealed onto a face panel portion 2. Numeral 1e denotes a junction between the neck portion 1a and the cone portion 1b, generally referred to as "neck seal line". The neck seal line has a glass wall thickness slightly smaller than those of other portions and, hence, is inferior to other portions in regard to strength. The above mentioned portions in cooperation form a glass bulb 20.
An electron gun 3 is sealed to the neck portion 1a. An explosion proof band 4 is wound on the side faces on the face panel portion 2 so as to provide explosion proof nature. A silicone resin film 5 is formed around the high voltage anode button 1d on the funnel main body portion 1c for the purpose of electrical insulation. A conductive film 6 for imparting electrostatic capacitance to the cathode ray tube is formed on the outer surface of the funnel main body portion 1c. Usually, the conductive film 6 is formed by applying, for example, a paste of graphite and may be discontinuous when viewed microscopically. Numeral 28 designates a straight line which is parallel to the neck 1a and which constitutes the axis of the cathode ray tube.
The cathode ray tube device thus formed has, as shown in FIG. 5, a deflection yoke 7 which is secured to a portion of the glass bulb between the cone portion 1b and the neck portion 1a and which serves to deflect the electron beam. Referring to FIG. 6, the deflection yoke 7 is composed of a horizontal deflection coil 7a, a vertical deflection coil 7b and a deflection yoke main body portion 7c. The fixing of the deflection yoke 7 to the funnel portion 1 is achieved by means of a fixing band 10 which is provided on a portion of the deflection yoke 7 adjacent to the neck portion 1a. A gap generally referred to as "self convergence system" is provided between the deflection yoke 7 and the funnel portion 1 for the purpose of adjustment of the convergence characteristics. The size of this gap is adjustable by oscillation of the deflection yoke 7 around the fixing band 10. This operation will be referred to as "oscillation convergence adjustment", hereinafter.
In operation, an electron beam emitted from the electron gun 3 sealed to the neck portion 1a is deflected both in the horizontal and vertical directions by means of the horizontal deflection coil 7a and the vertical deflection coil 7b of the deflection yoke 7, so as to scan a fluorescent film formed on the inner side of the face panel portion 2, thereby forming a desired image. The amplitude of the deflection of the electron beam is in inverse proportion to the square root of the voltage applied to the anode button 1d mentioned above.
In recent years, undesirable effect of electromagnetic waves on human bodies has been noticed as a problem to be overcome. This problem has been recognized also in display monitors. Namely, there is a fear that the human bodies are adversely affected by alternating electric field generated mainly by deflection yoke, i.e., electric field radiated from the deflection yoke. Based upon such recognition, National Conference of Measurement and Test of Sweden (MPR), as well as Central Labor Conference of Sweden (TCO), has proposed standards concerning electromagnetic waves emitted from display monitors, as shown in Table 1 below.
TABLE 1 ______________________________________ ELF band VLF band Measuring condition ______________________________________ 5 Hz to 2 KHz 2.5 V/m or 50 cm apart from CRT MPR less face, 20.degree. C., 21% humidity TCO 10 V/m or 1.0 V/m or 30 cm apart from CRT less less face, 20.degree. C., 21% humidity ______________________________________
Thus, the known cathode ray tube devices have no measure for shielding alternating electric field which is radiated from the deflection yoke. The present inventors have measured the alternating electric field at VLF band on a cathode ray tube of 16 inch size and having an anticharge coating of 2.6.times.109 W on face plate to obtain the results as shown in Table 2 below.
TABLE 2 ______________________________________ Type of CRT Anticharge treated CRT Measuring method MPR11 TCO ______________________________________ AC filed Hor. Freq. 31 KHz 2.3 V/m 5.0 V/m VLF band Hor. Freq. 45 KHz 3.4 V/m 8.3 V/m (V/m) Hor. Freq. 64 KHz 4.8 V/m 12.0 V/m ______________________________________
From Table 2, it is understood that the intensity of alternating electric field at VLF band has a dependency on horizontal frequency. More specifically, when the horizontal frequency is elevated to cope with a demand for higher resolution, the alternating electric field at VLF band is correspondingly increased. Consequently, alternating electric field of an intensity exceeding the standard levels shown in Table 1 penetrates the funnel portion and the face panel of the CRT so as to adversely affect the user's body.
As a countermeasure for obviating this problem, an arrangement is proposed in Japanese patent Laid Open No. 3116902 in which a grounded conductive film is formed on a predetermined area of the surface of the funnel portion and the deflection yoke is mounted via an insulating member which covers the conductive film.
This arrangement, however, poses a problem in that a gap is often formed between the conductive film on the cone portion and the insulating member due to dimensional error of the cone portion and the insulating member incurred during manufacturing of the cathode ray tube device, tending to degrade insulation due to accumulation of dust and other foreign matters in the gap during long use of the cathode ray tube device.